Long chain olefins (C10+) are important starting materials in the production of sulfonate surfactants, in which the olefins are used to alkylate aromatic hydrocarbons and the resultant alkyl aromatics are sulfonated to produce alkylaryl sulfonates. In addition, the alcohols of long chain olefins have considerable commercial importance in a variety of applications, including detergents, soaps, surfactants, and freeze point depressants in lubricating oils. In such applications, it is important that the olefins employed are substantially free of quaternary carbon atoms because materials containing quaternary carbon atoms are resistant to biodegradation.
One potential route for the production of long chain olefins is by the oligomerization of lower (C2 to C6) olefins, typically using an acidic catalyst, such as a zeolite. Thus, for example, it is known from U.S. Pat. Nos. 3,960,978, 4,150,062; 4,211,640; 4,227,992; and 4,547,613 to oligomerize lower olefins over ZSM-5.
U.S. Pat. Nos. 4,855,527; 4,870,038 and 5,026,933 describe a process for producing high molecular weight, slightly branched hydrocarbon oligomers from a lower olefin feedstock employing a shape selective crystalline silicate catalyst, ZSM-23, which has been surface deactivated. The resultant oligomer mixture comprises at least 20% by weight of olefins having at least 12 carbon atoms and an average of from 0.8 to 2.0 branches per carbon chain.
U.S. Pat. No. 5,284,989 is directed to a process for producing substantially linear hydrocarbons by oligomerizing a lower olefin at elevated temperature and pressure in the presence of an acidic aluminosilicate zeolite selected from ZSM-22, ZSM-23 and ZSM-35 which has been surface-deactivated by contacting with oxalic acid.
In view of the need to avoid the production of quaternary carbon atoms in the resultant olefin oligomers, it is normal to employ oligomerization feeds which consist essentially of n-olefins and which are substantially free of iso-olefins, such as iso-butylene and iso-amylene. This poses a problem in that one source of lower olefins in a modem integrated oil refinery is the unreacted effluent stream from the MTBE (methyl tertiary butyl ether) production unit, which stream inherently contains up to 5 wt % of iso-butylene. Thus, existing oligomerization processes either avoid the use of the MTBE effluent feed or else require expensive purification steps to remove the iso-olefins.
In accordance with the invention, it has now surprisingly been found that, when surface deactivated ZSM-23 is used to oligomerize a lower olefin feed containing significant quantities of iso-olefins, such as the unreacted effluent from an MTBE unit, the C12+ product is substantially free of quaternary carbon atoms. Instead, it is found that any quaternary carbon-containing materials are concentrated in the C8 fraction, which can then be removed for use as a high-octane gasoline product. Although the reason for this desirable result is not fully understood, it is believed that the size of the pores of the ZSM-23 are such that, although iso-butylene can enter the pores to react with, for example, n-butylene, the resultant branched C8 oligomer is too large to access the pores for further reaction.